Spark ignition engines typically have a gas pedal that is mechanically connected to an air throttle that meters air into the engine. Stepping on the gas pedal typically results in opening an air throttle, which allows more air into the engine. In some cases, a fuel injector controller adjusts the fuel that is provided to the engine to maintain a desired air/fuel ratio (AFR). The AFR is usually held close to a stoichiometric ratio (e.g. 14.6:1) to produce stoichiometric combustion, which helps minimize engine emissions and allows three-way catalysts to simultaneously remove hydrocarbons, carbon monoxide, and oxides of nitrogen (NOX).
In contrast, compression ignition engines such as diesel engines do not usually operate at stoichiometric ratios, and thus typically result in greater emissions with different emission components. Because of recent increases in the use of diesel engines in the automotive and light truck markets, federal regulations have been passed requiring more stringent emission levels for diesel engines. Such regulations have prompted automakers to consider alternative methods for improving engine efficiency and reducing emissions.
Unlike spark ignition engines, the fuel pedal of a diesel engine is typically not directly connected to an air throttle that meters air into the engine. Instead, in those diesel engines equipped with electronic fuel injection (EFI), pedal position is often sensed by a pedal position sensor that senses pedal position and adjusts the fuel rate provided to the engine, allowing more or less fuel per fuel pump shot to be provided to the engine. In many modern diesel engines, the air to the engine is controlled by a turbocharger such as a Variable Nozzle Turbocharger (VNT) or waste-gate turbocharger. Typically, there is a time delay or “turbo lag” between when the operator engages the fuel pedal to inject more fuel and when the turbocharger spins-up to provide the additional air required to produce the desired AFR. This “turbo-lag” can reduce the responsiveness and performance of the engine, and can increase the amount of emissions discharged from the engine.
There are typically no sensors in the exhaust stream of a diesel engine that are analogous to those emissions sensors found in spark ignition engines. One reason for their absence is that diesel engines generally operate at about twice as lean as spark ignition engines. As such, the oxygen level in the exhaust of a diesel engine can be at a level where standard oxygen emission sensors do not provide useful information. At the same time, diesel engines typically burn too lean for conventional three-way catalysts. As a result, control over combustion in a diesel engine is typically performed in an “open-loop” manner, often relying on engine maps or the like to generate set points for the intake manifold parameters that are believed to be favorable for acceptable exhaust emissions.